We use MR using Pig (data in cassandra/CFS) with a 6 node hadoop cluster to process timeseries data. The events contain user metrics like which view was tapped, user behavior, search result, clicks etc.

We process these events to use it downstream for our search relevancy, internal metrics, see top products.

We did this on mysql for a long time but things went really slow. We could have optimized mysql for performance but cassandra was an easier way to go about it and it works for us for now.